[Pw_forum] changing total energy due to changing ibrav

[Tram Bui]

Thank you for your respond. I just compare the total ground state energy
from two output files. I basically created two input files for the one
material structure (zinc blend). The calculation is done on 1x1x1 unit
cell, so I used all parameters the same for both input file except the
ibrav between 0 and 2. On the ibrav=2, then it is simple, I just have to
list two atom positions for zinc blend structure (000 and 1/4 1/4 1/4). For
the ibrav=0, I have to list 8 atom positions (4 for the fcc lattice and 4
for the tetrahedral sites). lattice parameter is the same, k-point and
cutoff energy is also the same. The calculated total energy value I got
from ibrav=0 is 4 times higher than that from ibrav=2. I had the idea of
why it is the case but I just want to make sure I understand it correctly.
So if you find anything wrong on the way i set up my file, please let me
know. Please also accept my apology if my question confused you.

Thank you,

On Wed, Jan 16, 2013 at 7:13 AM, Duy Le <ttduyle at gmail.com> wrote:

> Should you compare total energy per atom or total energy per unit volume?
> Of course, you have to use well-converged cutoff and k-point sampling.
> ----------------------------------------------------
> Duy Le
> Postdoctoral Associate
> Department of Physics
> University of Central Florida.
> Website: http://www.physics.ucf.edu/~dle
>
>
> On Wed, Jan 16, 2013 at 3:25 AM, Lorenzo Paulatto <
> lorenzo.paulatto at impmc.upmc.fr> wrote:
>
>> On 16 January 2013 02:33, Tram Bui <trambui at u.boisestate.edu> wrote:
>>
>>> Dear QE Users,
>>>        I ran the calculation for a simple 1x1x1 unit cell SiC (zinc
>>> blend structure) . The calculated total energy for when ibrav=2 (two atoms
>>> per cell) is different from when ibrav=0 (8 atoms per cell). Would you give
>>> me some information on why the total energy changes?
>>>
>>
>>
>> Dear Tram,
>> maybe you did not use 2 sets of k-points that map perfectly from one
>> system to the other.
>>
>> bests
>>
>> --
>> Dr. Lorenzo Paulatto
>> IdR @ IMPMC -- CNRS & Université Paris 6
>> phone: +33 (0)1 44275 084 / skype: paulatz
>> www:   http://www-int.impmc.upmc.fr/~paulatto/
>> mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05
>>
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-- 
Tram Bui

M.S. Materials Science & Engineering
trambui at u.boisestate.edu
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